Hello - One thing I like about answering support questions is that I learn new things. I have not used “inner region” before, but in looking at your model I noticed a few things that may be giving you difficulty.
For one, there are two gaps in you mesh between the nostrils. This is easy to see if, in solid view, you select Face Orientation fro the Overlay drop down menu.
I noticed the gaps because I was checking the normals, and found that some teeth, parts of the eyes, and the top and bottom of the mouth were “inside” faces. When I pressed SHIFT-N to recalculate normals, there were still some parts considered inner faces (one of the teeth, for instance).
Lastly, you have a lot of triangles (3-sided faces) and N-gons (polygons with >4 sides). I cannot say for sure, but this might be the root of you problem. For the teeth, for instance, I would make them separate from the body. Make cylinders and scale in one end, then use Grid Fill to fill with faces. This would simplify your mesh and make it easier to use only quads (4-sided polygons).